Tweak marker selection snackbar messages so they are more suitable for filter-related scenarios.

[bobular]

          I just noticed that the snackbar messages come up as a result of filtering. This was an inevitability of my simple approach - oops!

Example 1.

• Zoom in to Southern Africa + Madagascar
• Select markers in Madagascar then add a filter for country=Zimbabwe
• The Selected markers that went off-screen have been deselected message appears.

This could be ameliorated with some minor verbiage changes. The code that produces the message doesn't know about filters and subsets AFAIK.

Example 2.

• Select some markers
• Add a filter that removes all markers from the screen (e.g. country = Poland)
• The Marker selection has been cancelled because aggregation level has changed due to zooming message appears.

This is coming up because consolidatedMarkers.length is zero. This can be fixed with a new message for when there are no markers on the screen any more.

Originally posted by @bobular in https://github.com/VEuPathDB/web-monorepo/issues/617#issuecomment-2079398286

[bobular]

The new message could be Marker selection has been cancelled because there are no visible markers in the current view

[bobular]

The tweaked message could be Selected markers that are no longer visible have been deselected